Question

8 analyze this pedigree. if ▣ represents a male with the homozygous recessive genotype (aa), what is the mother’s genotype? (pedigree image: a purple square (aa male) and a yellow circle (mother) have three children: purple square (aa), yellow square, yellow circle)

Explanation:

Step1: Recall genotype inheritance

In Mendelian genetics, offspring inherit one allele from each parent. The affected male (purple square) has genotype \( aa \), so he got one \( a \) from each parent.

Step2: Analyze father's genotype

The father is also an affected male (purple square), so his genotype is \( aa \). He can only pass on \( a \) alleles.

Step3: Determine mother's contribution

The affected son has \( aa \), one \( a \) from father (\( aa \)) and one \( a \) from mother. But there are unaffected children (yellow squares and circle), which means the mother must carry a dominant allele (\( A \)) to pass to unaffected offspring. Since she is unaffected (yellow circle), her genotype must be \( Aa \) (because if she were \( AA \), all offspring would get an \( A \) from her and with father's \( a \), they'd be \( Aa \) and not show the recessive trait, but we have an affected child, so she can't be \( AA \); if she were \( aa \), she'd be affected, but she's not, so she must be \( Aa \)).

Answer:

\( Aa \)